How are Q# programs structured?

6 minutes

The Q# quantum programming language has a well-defined structure. In this unit, you'll review the main components of a Q# program.

Namespace

Every Q# file starts with a namespace. Here's an example:

namespace HelloQuantum {
    // Your code goes here.
}

The namespace keyword is used to define a namespace. Namespaces are used to organize Q# code into logical units. Their use becomes important when you use Q# libraries in your programs and when you write your own libraries.

Libraries

Q# libraries contain functions and operations that you can use in quantum programs. When you call a function or operation from a library, you use the open directive and specify the library's namespace. For example:

namespace HelloQuantum {

    open Microsoft.Quantum.Intrinsic;

    @EntryPoint()
    operation HelloQ() : Unit {
        Message("Hello quantum world!");
    }
}

In this example, the open directive tells the Q# compiler to look for the Message operation in the Microsoft.Quantum.Intrinsic namespace.

Entry points

EntryPoint tells the Q# compiler where to begin executing the program. Every Q# program must have at least one entry point followed by an operation.

Allocating qubits

In Q#, to obtain a qubit you use the use keyword. Every qubit you allocate with the use keyword starts in the |0〉 state.

You can allocate one or many qubits at a time. Here's an example that allocates one and five qubits:

use q1 = Qubit(); // Allocate one qubit
use q5 = Qubit[5]; // Allocate five qubits

Quantum operations

Operations are the basic building blocks of a Q# program. A Q# operation is a quantum subroutine. That is, it's a callable routine that contains quantum operations that modify the state of the qubit register.

To define a Q# operation, you specify a name for the operation along with its inputs and its output. Here's a basic example:

@EntryPoint()
operation SayHelloQ() : Unit {
    Message("Hello quantum world!");
}

Here, SayHelloQ is the name of the operation. It takes zero arguments as its input and returns type Unit, which means that the operation returns no information.

Q# libraries also provide operations that you can use in your programs. One operation you'll use later is the H operation. Given a qubit in Z-basis, the H operation puts the qubit into an even superposition. Once in superposition, the qubit has a 50% chance of being measured as zero or one.

Types

Q# provides many built-in types you're already familiar with, including Int, Double, Bool, and String, along with types that are specific to quantum computing. Q# also provides types that define ranges, arrays, and tuples. You can even define your own custom types.

In this module, you'll work with the Result type. A Result represents the result of a qubit measurement and can have one of two possible values: One and Zero.

Measuring qubits

In Q#, Pauli measurements are done by applying the Measure operation, which performs a joint measurement of one or more qubits in the specified Pauli bases. The Measure operation returns a Result type, that is either One or Zero.

Note

If the basis array and qubit array are different lengths, then the Measure operation will fail.

To implement a measurement in the computational basis {|0〉 ,|1〉} you can also use the M operation, which performs a measurement of a single qubit in the Pauli Z basis. Therefore the M operation is equivalent to applying Measure([PauliZ], [qubit]).

A simple example is the following program, which allocates one qubit in the |0〉 state, then applies a Hadamard operation H to it and measures the result in the PauliZ basis.

@EntryPoint()
operation MeasureOneQubit() : Result {
    // Allocate a qubit, by default it is in zero state      
    use q = Qubit();  
    // We apply a Hadamard operation H to the state
    // It now has a 50% chance of being measured 0 or 1  
    H(q);      
    // Now we measure the qubit in Z-basis.
    let result = M(q);

    // Reset the qubit so it can be safely released.
    Reset(qubit);
    return result;
}